Before discussing thermal insulation in more detail, it is useful to define the specific heat capacity of a body.
The heat capacity C of a substance is the quotient of the heat Q which is supplied to the body and the temperature increase ΔT that this causes:
                    C        =                              Q                          Δ              ⁢                                                          ⁢              T                                .                                    (        1        )            
The specific heat capacity c, also referred to as specific heat, arises from scaling the heat capacity C to the mass of the substance. In other words, the specific heat capacity of a substance is the energy that is needed to heat 1 kg of this substance by 1 K:
                    c        =                              C            m                    =                                    Q                                                m                  ·                  Δ                                ⁢                                                                  ⁢                T                                      .                                              (        2        )            
The specific heat capacity is only slightly dependent on the temperature. Since the specific heat capacity c is typically given as a material constant, the formula (2) is often written asQ=c·m·ΔT.  (3).
In thermal insulation, for example, with exhaust gas systems of vehicles, three mechanisms are basically to be considered, heat conduction, heat radiation and convection.
Heat conduction, also referred to as heat diffusion, is understood to be a heat flow as a result of a temperature difference. The direction of flow is according to the second law of thermodynamics always directed from the higher to the lower temperature. Ideally no heat energy is lost there.
With heat radiation, also electromagnetic radiation is emitted from a solid body, fluids or plasma. The radiated power P emitted is there proportional to the fourth power of the radiating body, i.e. P∝T4 (Stefan-Boltzmann law). In a vacuum, heat radiation is the only way to transfer thermal energy.
Convection or heat transfer is another mechanism for heat transport. Convection is caused by a flow which transports particles. For example, a fluid flowing can absorb heat from a surface or release it thereto. One cause of the transporting flow can be temperature differences. In forced convection, the particle transport is caused by external influence, for example, a fan or a pump. In natural convection, the particle transport is caused by a temperature gradient present within the medium.
In systems to be insulated, motors are often driven at high speed whereby a high noise level can occur. Therefore, the aspect of sound insulation is added to thermal insulation.
Three known types of insulation known from practice are described below by way of example.
1. Air Gap Insulation
A first example is air gap insulation without convection. In air gap insulation without convection, a self-contained insulating system is given in which substantially still air is used as insulating material. An advantage of this type of isolation is the low specific heat capacity of air. The system can therefore absorb only a small amount of heat. Due to this circumstance, the heat energy in heat-saturated insulating material, i.e. heat-saturated air, largely remains available to the system to be insulated. Furthermore, the heat loss by conduction is kept very low due to the low thermal conductivity of air. A drawback of this method is heat radiation which can act upon the surrounding components. Furthermore, only a very small temperature difference over the insulation zone is possible. This means that the surface temperature of the insulation system, i.e. the air, is only slightly lower than the application temperature, i.e. the temperature of the system to be insulated, such as the exhaust gas component. It can in summary be said about air gap insulation without convection that it offers very good energy conservation in the system to be insulated but for surrounding components does not ensure adequate protection due to the high surface temperatures. The environment is also barely protected against acoustic emissions.
2. Air Gap Insulation with Convection and a Heat Shield
A further example is air gap insulation with convection and a heat shield. Air gap insulation with convection and a heat shield, conventionally only referred to as a heat shield, is an insulation system which is not self-contained. It offers extended influence by the environment. In this type of insulation, a kind of shield is in a spaced manner attached in front of the component to be insulated and is to absorb the heat radiation in front of surrounding components in order to protect them. The drawbacks and advantages described above in 1. are here given in the reverse position. Due to the constant layer change of air, the heat emitted by the system to be insulated is constantly again absorbed, this is also in accordance with equation (3). This leads to increased energy losses in this system to be insulated because the amount of heat of the insulation to be absorbed, i.e. of the air gap, can virtually never approach zero.
Unlike with the air gap insulation without convection, a far lower temperature than the initial temperature is measured due to the circumstance of the continuously changing layers of air and the associated heat dissipation to the surface of the insulation system. Moreover, exposure of surrounding components to thermal radiation is greatly reduced due to the function of the shield. One drawback of this system is low acoustic absorption. It can in summary be said about air gap insulation with convection that surrounding components are very well protected against thermal influences, but that this is at the expense of energy conservation in the system to be insulated and of avoidance of acoustic emissions into the environment.
3. Isolation with Filling Material
A third example of insulation systems are those in which the insulating material is filling material between the system to be insulated and a metal outer shell. In high temperature applications, this is usually glass fibers, for example, silica fibers or ceramic fibers which are applied directly onto the systems to be insulated. This is currently one of the methods most commonly used in the automotive sector. This insulation method provides an in-between solution to the first two. The advantages are low surface temperatures at a small installation space, as well as good acoustic absorption by the fiber material. In terms of energy conservation in the system to be insulated, this is an in-between solution. No perceivable convection is given which is why it is in terms of energy conservation better suited than the heat shield insulation. However, this system provides a much greater amount of heat to be absorbed than the air gap insulation without convection, which in the present case means reduced energy conservation. A further drawback in terms of energy conservation is the lack of reflection of radiation caused by the direct application which allows only for heat conduction. It can in summary be said that this insulation system is the middle ground between the first two systems mentioned and therefore represents one of the most common methods used.